CD1283 Datasheet, PDF (56/94 Pages) Intel Corporation – IEEE 1284-Compatible Parallel Interface

CD1283 — IEEE 1284-Compatible Parallel Interface
7.2
Virtual Registers
The CD1283 has two operational contexts: a normal context that allows host access to most
registers and any channel, and a service-acknowledge context, allowing host access to some
registers specific to the channel requesting service. This special set of registers is called ‘virtual’
because they are only available to host access and are valid during this service-acknowledge
context; at all other times, their contents will be undefined and must not be written to by host
software.
The use of Virtual registers and context switching allows the CD1283 to maintain all channel-
specific information. The host need not make any changes to chip registers to access the registers
pertinent to the parallel channel.
The service-acknowledge context is entered in one of two ways: either through activation of the
SVCACKP* input pin (hardware activated), or through host software when the contents of any one
of PIR is copied into the AER by host software during a Poll-mode Acknowledge cycle (software-
activated). See Chapter 5.0 for a discussion of the differences between these two modes.
7.2.1
End-of-Service Request Register
Register Name: EOSRR
Register Description: End-of-Service Request
Access: Write only
Bit 7
Bit 6
Bit 5
X
X
X
Bit 4
X
Bit 3
X
Bit 2
X
8-Bit Hex Address: 60
Default Value: XX
Bit 1
X
Bit 0
X
The EOSRR is a ‘dummy’ location and is used to signal the end of a hardware-activated service-
acknowledge procedure, invoked by the activation of SVCACKP*. The data pattern written is a
‘don’t care’ value. Writing this location causes the CD1283 to perform its internal switch out of the
service-acknowledge context. This register is used only during a hardware-activated service
acknowledge and must not be written during Poll-mode operation.
7.2.2
Parallel Interrupt Vector Register
Register Name: PIVR
Register Description: Parallel Interrupt Vector
Access: Read only
Bit 7
Bit 6
Bit 5
Bit 4
User-Defined – Upper 5 Bits of LIVR
Bit 3
Bit 2
IT2
8-Bit Hex Address: 40
Default Value: 00
Bit 1
IT1
Bit 0
IT0
The value in this register is placed on the data bus, DB[7:0], when SVCACKP* is activated in
response to an active SVCREQP*. See Section 7.3.5 on page 60 for more details on the LIVR.
Table 10. PIVR[2:0] Encoding
IT2
IT1
IT0
0
0
0
Description
No active interrupt.
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Datasheet